INTERNET DRAFT Pat R. Calhoun
Category: Standards Track Sun Microsystems, Inc.
Title: draft-calhoun-diameter-proxy-01.txt William Bulley
Date: August 1998 Merit Network, Inc.
DIAMETER
Proxy Server Extensions
Status of this Memo
This document is an individual contribution for consideration by the
AAA Working Group of the Internet Engineering Task Force. Comments
should be submitted to the diameter@ipass.com mailing list.
Distribution of this memo is unlimited.
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at:
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at:
http://www.ietf.org/shadow.html.
Abstract
This DIAMETER Extension defines commands and AVPs that are used when
DIAMETER messages must be proxied by DIAMETER Servers. This extension
is intended to clearly define how proxying can be done with DIAMETER.
Calhoun, Bulley expires February 1999 [Page 1]
INTERNET DRAFT August 1998
Table of Contents
1.0 Introduction
1.1 Definitions
1.2 Terminology
2.0 Command Codes
2.1 Domain-Discovery-Request (DDR)
2.2 Domain-Discovery-Answer (DDA)
3.0 DIAMETER AVPs
3.1 Proxy-State
3.2 Digital-Signature
3.3 X509-Certificate
3.4 X509-Certificate-URL
3.5 Next-Hop
4.0 Protocol Definition
4.1 Data Integrity
4.1.1 Using Digital Signatures
4.1.1 Using Mixed Data Integrity AVPs
4.2 AVP Data Encryption
4.2.1 AVP Encryption with Public Keys
4.3 Public Key Cryptography Support
4.3.1 X509-Certificate
4.3.2 X509-Certificate-URL
4.3.3 Static Public Key Configuration
5.0 References
6.0 Acknowledgements
7.0 Author's Address
1.0 Introduction
Many services, including ROAMOPS and MobileIP, have a requirement for
DIAMETER Server to proxy a request to another DIAMETER Server. The
concept of proxying AAA requests was introduced by RADIUS and has
been in use for many years.
Unfortunately due to the fact that RADIUS only supports hop-by-hop
security, where each node has a security association with the next
hop, this does introduce some security flaws. Specifically a
fraudulent proxy server can modify some portions of an AAA request in
order to make the next hop improperly believe that some services were
rendered. For example, a DIAMETER Proxy Server could modify an
accounting request, such as the number of bytes that a user
transfered, and the end system would have no way of determining that
this change occured.
1.1 Definitions
Calhoun, Bulley expires February 1999 [Page 2]
INTERNET DRAFT August 1998
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized.
MUST This word, or the adjective "required", means that the
definition is an absolute requirement of the
specification.
MUST NOT This phrase means that the definition is an absolute
prohibition of the specification.
SHOULD This word, or the adjective "recommended", means that
there may exist valid reasons in particular circumstances
to ignore this item, but the full implications must be
understood and carefully weighed before choosing a
different course.
MAY This word, or the adjective "optional", means that this
item is one of an allowed set of alternatives. An
implementation which does not include this option MUST
be prepared to interoperate with another implementation
which does include the option.
2.0 Command Codes
This document defines the following DIAMETER Commands. All DIAMETER
implementations supporting this extension MUST support all of the
following commands:
Command Name Command Code
-----------------------------------
Domain-Discovery-Request 261
Domain-Discovery-Answer 262
2.1 Domain-Discovery-Request (DDR)
Description
The Domain-Discovery-Request message is used by a DIAMETER device
wishing to get contact information about a domain's home
authentication server as well as to receive password policy
information. This message MUST contain the User-Name attribute in
order to pass along the user's domain information.
It is not necessary for an implementation to issue a DDR in order
to make use of a proxy server.
Calhoun, Bulley expires February 1999 [Page 3]
INTERNET DRAFT August 1998
The X509-Certificate or the X509-Certificate-URL [2] MUST be
present in this message in order to inform the home authentication
server of the issuing host's certificate.
At least one Extension-Id AVP MUST be present in the DDR in order
to inform the peer about the locally supported extensions.
Message Format
<Domain-Discovery-Req> ::= <DIAMETER Header>
<Domain-Discovery-Req Command AVP>
<Host-IP-Address AVP>
[<Host-Name AVP>]
<Extension-Id AVPs>
<User-Name AVP>
[<X509-Certificate AVP>]
[<X509-Certificate-URL AVP>]
<Timestamp AVP>
<Nonce AVP>
{<Integrity-Check-Vector AVP> ||
<Digital-Signature AVP }
AVP Format
A summary of the Domain-Discovery-Request packet format is shown
below. The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AVP Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AVP Length | Reserved |P|T|V|E|H|M|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Command Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
AVP Code
256 DIAMETER Command
AVP Length
The length of this attribute MUST be 12.
AVP Flags
The 'M' bit MUST be set. The 'H' and 'E' MAY be set depending
Calhoun, Bulley expires February 1999 [Page 4]
INTERNET DRAFT August 1998
upon the security model used. The 'V', 'T' and the 'P' bits
MUST NOT be set.
Command Type
The Command Type field MUST be set to 261 (Domain-Discovery-
Request).
2.2 Domain-Discovery-Answer (DDA)
Description
The Domain-Discovery-Answer message is sent in response to the
Domain-Discovery-Request message by the domain's Home
authentication server. The message MUST contain either the Host-
Name or Host-IP-Address and either the X509-Certificate or the
X509-Certificate-URL attribute and SHOULD contain at least one
Framed-Password-Policy AVP.
At least one Extension-Id AVP MUST be present in the DDA in order
to inform the requestor about the locally supported extensions.
The Domain-Discovery-Answer message MUST include the Result-Code
AVP to indicate whether the request was successful or not. The
following Error Codes are defined for this command:
DIAMETER_ERROR_UNKNOWN_DOMAIN 1
This error code is used to indicate to the initiator of the
request that the requested domain is unknown and cannot be
resolved.
DIAMETER_ERROR_BAD_CERT 2
This error code is used to indicate that the X509-
Certificate or the X509-Certificate-URL in the Domain-
Discovery-Request was invalid.
DIAMETER_ERROR_CANNOT_REPLY 3
This error code is returned when either an intermediate
DIAMETER node or the home authentication server cannot reply
to DIAMETER messages directly. This could be that the
policy of an intermediate DIAMETER server does not permit
direct contact and therefore requires proxying. It could
also signify that the home authentication server does not
have public key support.
Message Format
Calhoun, Bulley expires February 1999 [Page 5]
INTERNET DRAFT August 1998
<Domain-Discovery-Answer> ::= <DIAMETER Header>
<Domain-Disc-Answer Command AVP>
<Result-Code AVP>
[<Error-Code AVP>]
<Host-IP-Address AVP>
[<Host-Name AVP>]
<Extension-Id AVPs>
<Framed-Password-Policy AVP>
[<X509-Certificate AVP>]
[<X509-Certificate-URL AVP>]
<Timestamp AVP>
<Nonce AVP>
{<Integrity-Check-Vector AVP> ||
<Digital-Signature AVP }
AVP Format
A summary of the Domain-Discovery-Answer packet format is shown
below. The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AVP Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AVP Length | Reserved |P|T|V|E|H|M|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Command Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
AVP Code
256 DIAMETER Command
AVP Length
The length of this attribute MUST be 12.
AVP Flags
The 'M' bit MUST be set. The 'H' and 'E' MAY be set depending
upon the security model used. The 'V', 'T' and the 'P' bits
MUST NOT be set.
Command Type
The Command Type field MUST be set to 262 (Domain-Discovery-
Answer).
Calhoun, Bulley expires February 1999 [Page 6]
INTERNET DRAFT August 1998
3.0 DIAMETER AVPs
This section will define the mandatory AVPs which MUST be supported
by all DIAMETER implementations. Note the first 256 AVP numbers are
reserved for RADIUS compatibility.
The following AVPs are defined in this document:
Attribute Name Attribute Code
-----------------------------------
Proxy-State 33
Digital-Signature 260
X509-Certificate 264
X509-Certificate-URL 265
Next-Hop 278
3.1 Proxy-State
Description
The Proxy-State AVP is used by proxy servers when forwarding
requests and contains opaque data that is used by the proxy to
further process the response.
This attribute should be removed by the proxy server before the
response is forwarded to the NAS, and SHOULD therefore not be
protected by the Integrity-Check-Vector or the Digital-Signature.
AVP Format
A summary of the Proxy-State AVP format is shown below. The fields
are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AVP Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AVP Length | Reserved |P|T|V|E|H|M|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+-+-+-+-+
AVP Code
33 for Proxy-State.
Calhoun, Bulley expires February 1999 [Page 7]
INTERNET DRAFT August 1998
AVP Length
The length of this attribute MUST be at least 9.
AVP Flags
The 'M' bit MUST be set. The 'H' and 'E' MAY be set depending
upon the security model used. The 'V', 'T' and the 'P' bits
MUST NOT be set.
String
The String field is one or more octets. The actual format of
the information is site or application specific, and a robust
implementation SHOULD support the field as undistinguished
octets.
3.2 Digital-Signature
Description
The Digital-Signature AVP is used for authentication, integrity as
well as non-repudiation. A DIAMETER entity adding AVPs to a
message MUST ensure that all AVPs appear prior to the Digital-
Signature AVP (with the exception of the Integrity-Check-Vector
AVP that MUST appear after the Digital-Signature AVP). The
Timestamp AVP MUST be present to provide replay protection and the
Nonce AVP must be present to add randomness to the packet.
The DIAMETER header as well as all AVPs with the 'P' bit disabled
are protected by the Digital-Signature.
In order to support proxy DIAMETER servers, which forwards
messages to next hop server, the proxy server MUST NOT modify any
AVPs with the 'P' bit disabled. This ensures that end-to-end
security is maintained even through proxy arrangements.
The Digital-Signature is generated in the method described in
section 4.5.2.
All DIAMETER implementations supporting this extension MUST
support this AVP.
AVP Format
A summary of the Digital-Signature AVP format is shown below. The
fields are transmitted from left to right.
Calhoun, Bulley expires February 1999 [Page 8]
INTERNET DRAFT August 1998
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AVP Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AVP Length | Reserved |P|T|V|E|H|M|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Transform ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+-+-+-+-+
AVP Code
260 Digital-Signature
AVP Length
The length of this attribute MUST be at least 17.
AVP Flags
The 'M' bit MUST be set. The 'H' MAY be set if the request is
protected with an ICV AVP. The 'E', 'V', 'T' and the 'P' bits
MUST NOT be set.
Address
The Address field contains the IP address of the DIAMETER host
which generated the Digital-Signature.
Transform ID
The Transform ID field contains a value that identifies the
transform that was used to compute the signature. The following
values are defined in this document:
RSA [9] 1
Data
The Data field contains the digital signature of the packet up
to this AVP.
3.3 X509-Certificate
Calhoun, Bulley expires February 1999 [Page 9]
INTERNET DRAFT August 1998
Description
The X509-Certificate is used in order to send a DIAMETER peer the
local system's X.509 certificate chain, which is used in order to
validate the Digital-Signature attribute.
Section 4.3.1 contains more information about the use of
certificates.
AVP Format
A summary of the X509-Certificate AVP format is shown below. The
fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AVP Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AVP Length | Reserved |P|T|V|E|H|M|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+-+-+-+-+
AVP Code
264 X509-Certificate
AVP Length
The length of this attribute MUST be at least 9.
AVP Flags
The 'M' bit MUST be set. The 'H' and 'E' MAY be set depending
upon the security model used. The 'V', 'T' and the 'P' bits
MUST NOT be set.
Data
The Data field contains the X.509 Certificate Chain.
3.4 X509-Certificate-URL
Description
The X509-Certificate-URL is used in order to send a DIAMETER peer
Calhoun, Bulley expires February 1999 [Page 10]
INTERNET DRAFT August 1998
a URL to the local system's X.509 certificate chain, which is used
in order to validate the Digital-Signature attribute.
Section 4.3.1 contains more information about the use of
certificates.
AVP Format
A summary of the X509-Certificate-URL AVP format is shown below.
The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AVP Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AVP Length | Reserved |P|T|V|E|H|M|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| String ...
+-+-+-+-+-+-+-+-+
AVP Code
265 X509-Certificate-URL
AVP Length
The length of this attribute MUST be at least 9.
AVP Flags
The 'M' bit MUST be set. The 'H' and 'E' MAY be set depending
upon the security model used. The 'V', 'T' and the 'P' bits
MUST NOT be set.
String
The String field contains the X.509 Certificate Chain URL.
3.5 Next-Hop
Description
The Next-Hop AVP MUST preceed a Digital-Signature AVP and is used
to validate that a packet traversed the proxy chain that was
intended. A DIAMETER message that includes a Host-IP-Address with
a different Next-Hop AVP that preceeds it is considered invalid.
Calhoun, Bulley expires February 1999 [Page 11]
INTERNET DRAFT August 1998
AVP Format
A summary of the Next-Hop AVP format is shown below. The fields
are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AVP Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AVP Length | Reserved |P|T|V|E|H|M|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
AVP Code
278 Next-Hop
AVP Length
The length of this attribute MUST be 12.
AVP Flags
The 'M' bit MUST be set. The 'H' and 'E' MAY be set depending
upon the security model used. The 'V', 'T' and the 'P' bits
MUST NOT be set.
Address
This field contains the IP Address of the next DIAMETER Server.
4.0 Protocol Definition
This section will describe how the base protocol works (or is at
least an attempt to).
4.1 DIAMETER Proxying
The ROAMOPS specification [11] discusses how RADIUS servers can be
arranged in a hierarchical manner, allowing message exchanges across
domain boundaries. The specification also describes some security
flaws encountered when RADIUS is used in a proxy environment. The
DIAMETER extension described in this document introduces end-to-end
security, which solves many of the problems encountered when RADIUS
Calhoun, Bulley expires February 1999 [Page 12]
INTERNET DRAFT August 1998
is used.
(Request) (Request)
+------+ -----> +------+ ------> +------+
| | | | | |
| NASB +--------------------+ DIA2 +--------------------+ DIA1 |
| | | | | |
+------+ <----- +------+ <------ +------+
(Answer) (Answer)
In this example NASB generates a Request that is forwarded to DIA2.
The Request contains a digital signature AVP which "protects" all
mandatory (or non-editable) AVPs within the request. All AVPs which
may be modified, or removed appear after the digital signature AVP.
Once DIA2 receives the request, it MAY authenticate the request to
ensure that it was originated by NASB (verifying the signature is not
necessary if the link between NASB and DIA2 is secured using IPSEC).
The DIA2 Server SHOULD add the Proxy-State AVP, which contains opaque
data that MUST be present in the response and is used to identify
state information related to the request or response. If the Proxy-
State AVP is already present in the request, it MUST be replaced with
DIA2's Proxy-State AVP. This means that the Proxy-State AVP cannot be
protected end-to-end. The Server MAY also add other new AVPs to the
request. All AVPs that are protected by the Digital-Signature MUST
have the 'P' bit set, and all AVPs MUST preceed the Digital-Signature
AVP. The message is then forwarded towards the DIA1 server.
Since all packets between NASB and DIA1 must flow through DIA2, it is
not possible to use IPSEC between them, therefore the digital
signature must be present within the DIAMETER protocol itself. NASB
and DIA1 do not need to validate the digital signature AVP if the
link between themselves and DIA2 is secured using IP Security.
This mechanism now provides a method for DIA1 to prove that NASB was
the initiator of the request (note that DIAMETER also includes a
timestap to prevent replay attacks). It also provides a method of
ensuring that DIA2 cannot modify any protected AVPs (such as length
of call, etc.).
In addition, this same mechanism can be used for end-to-end
encryption of AVPs. In the case where NASB needs to encrypt an AVP it
is done using asymetric encryption using DIA1's public key. This
ensures that only DIA1 can decrypt the AVP.
An attack has been identified in this proposal which allows a
malicous man in the middle attack as shown in the following diagram.
Calhoun, Bulley expires February 1999 [Page 13]
INTERNET DRAFT August 1998
(Request) (Request) (Request)
+------+ -----> +------+ -----> +------+ -----> +------+
| | | | | | | |
| NASB +----------+ DIA2 +----------+ DIA3 +----------+ DIA1 |
| | | | | | | |
+------+ <----- +------+ <----- +------+ <----- +------+
(Answer) (Answer) (Answer)
In this example, DIA3 traps packets generated from DIA2 towards DIA1,
removes the AVPs added by DIA2 and inserts its own AVPs (possibly by
trying to convince DIA1 to pay DIA3 for the services). This attack
can be prevented by supporting a new Next-Hop AVP. In this case when
NASB prepares a request it inserts a non-editable Next-Hop AVP which
contains DIA2's identitity. DIA2 also adds a Next-Hop AVP with DIA1
as the next hop.
This mechanism ensures that a man in the middle cannot alter the
packet by overriding the previous hop's additions and signature. DIA1
could easily validate the packet's path with the use of the Next-Hop
AVPs.
4.2 Domain Discovery
The Domain Discovery message set is very useful in determining the
Home authentication server, the password policies for the domain, as
a mechanism to retrieve a certificate (or a pointer to a
certificate).
Note that it is not necessary for a host to issue a Domain Discovery
in order to make use of a proxy. A DIAMETER Request MAY be proxied by
an intermediate server without the knowledge of the client, however
the client will be unable to validate any Digital-Signatures if the
home authentication server's certificate or public key is not known.
The following example shows a case where DIA1 needs to communicate
with DIA3. In this example it is necessary to use DIA2 as a proxy in
order for both ISPs to communicate. Although this MAY be desireable
in some business models, there are cases where it is beneficial to
remove the proxy altogether and allow both DIA3 and DIA1 to
communicate in a secure fashion.
Calhoun, Bulley expires February 1999 [Page 14]
INTERNET DRAFT August 1998
(DD-Request) (DD-Request)
+------+ -----> +------+ ------> +------+
| | | | | |
| DIA1 +--------------------+ DIA2 +--------------------+ DIA3 |
| | | | | |
+------+ <----- +------+ <------ +------+
(DD-Response) (DD-Response)
The way the Domain Discovery works is that prior to sending out an
authentication request DIA1 would issue a Domain Discovery message
towards DIA2. This message is protected with the digital signature as
well as the Next-Hop AVP. DIA2 would then forward the request to DIA3
including the Next-Hop and the digital signature AVP.
When DIA3 receives the request, it MUST save the certificate (or the
pointer to the certificate) and respond back including the local
password policy, DIA3's certificate, its contact information (i.e. IP
address) and protect the response with the digital signature.
Note that in all cases the TimeStamp AVP is also present to ensure no
replay packets are accepted.
When DIA2 receives the packet, it must add the Next-Hop AVP as well
as the digital signature AVP. When DIA1 receives the packet it then
knows a direct route to communicate with DIA3 since the contact
information is present in the response. The fact that both DIA1 and
DIA3 can now communicate directly allows both peers to use IPSEC to
protect the message exchange (it may be desirable to use the
Digital-Signature AVP in instances where records of digitally signed
packets must be kept).
(Request)
+------+ -----> +------+
| | | |
| DIA1 +--------------------+ DIA3 |
| | | |
+------+ <----- +------+
(Answer)
In addition, the password policy is also present which can indicate
whether DIA3 is willing to accept CHAP, PAP or EAP authentication.
Note that the Domain-Discovery-Request/Answer MUST include at least
one Extension-Id AVP [2].
4.3 Data Integrity
Calhoun, Bulley expires February 1999 [Page 15]
INTERNET DRAFT August 1998
This section will describe how data integrity and non-repudiation is
achieved using the Digital-Signature AVP.
Note that the Timestamp and Nonce AVPs MUST be present in the message
PRIOR to the Digital-Signature AVPs discussed in this section. The
Timestamp AVP provides replay protection and the Nonce AVP provides
randomness.
Any AVPs in a message that is not followed by either the ICV or the
Digital-Signature AVPs MUST be ignored.
4.3.1 Using Digital Signatures
In the case of a simple peer to peer relationship the use of IPSEC is
sufficient for data integrity and non-repudiation. However there are
instances where a peer must communicate with another peer through the
use of a proxy server. IPSEC does not provide a mechanism to protect
traffic when two peers must use an intermediary node to communicate
at the application layer therefore the Digital-Signature AVP MUST be
used.
The following diagram shows an example of a router initiating a
DIAMETER message to DIA1. Once DIA1 has finished processing the
message it adds its signature and forwards the message to the non-
trusted DIA2 proxy server. If DIA2 needs to add or change any
protected AVPs it SHOULD add its digital signature before forwarding
the message to DIA3.
+------+ -----> +------+ -----> +------+ -----> +------+
| | | | | non- | | |
|router+----------+ DIA1 +----------+trustd+----------+ DIA3 |
| | | | | DIA2 | | |
+------+ <----- +------+ <----- +------+ <----- +------+
Since some fields within the DIAMETER header will change "en route"
towards the final DIAMETER destination, it is necessary to set the
unprotected fields to zero (0) prior to calculating the signature.
The two unprotected fields are the identifier and the length in the
DIAMETER header.
The following is an example of a message that include end-to-end
security:
Calhoun, Bulley expires February 1999 [Page 16]
INTERNET DRAFT August 1998
<DIAMETER Message> ::= <DIAMETER Header>
<Command AVP>
[<Additional AVPs>]
<Next-Hop AVP>
<Timestamp AVP>
<Nonce AVP>
<Digital-Signature AVP>
The AVP Header's 'P' bit is used to identify which AVPs are
considered protected when applying a digital signature to a DIAMETER
message. Protected AVPs cannot be changed "en route" since they are
protected by the Digital Signature AVP. All AVPs added by a DIAMETER
entity MUST appear prior to the Digital Signature AVP that is added
(with the exception of the Integrity-Check-Vector AVP). However, only
AVPs with the 'P' bit set are used in the digital signature
calculation.
The Next-Hop AVP indicates the intended recipient of the DIAMETER
message. When a DIAMETER message is received with a Next-Hop AVP
that does not correspond with the Host-IP-Address that follows, the
message MUST be considered invalid and MUST be rejected.
The Data field of the Digital-Signature AVP contains the RSA/MD5
signature algorithm as defined in [9].
4.3.2 Using Mixed Data Integrity AVPs
The previous sections described the Integrity-Check-Vector and the
Digital-Signature AVP. Since the ICV offers hop-by-hop integrity and
the digital signature offers end to end integrity, it is possible to
use both AVPs within a single DIAMETER message.
The following diagram provides an example where DIAMETER Server 1
(DIA1) communicates with DIA3 using Digital-Signatures through DIA2.
In this example ICVs are used between DIA1 and DIA2 as well as
between DIA2 and DIA3.
<Public-Key>
----------------------------->
<Shared-Secret> <Shared-Secret>
+------+ -----> +------+ -----> +------+
| | | | | |
| DIA1 +----------+ DIA2 +----------+ DIA3 |
| | | | | |
+------+ +------+ +------+
Using the previous diagram, the following message would be sent
Calhoun, Bulley expires February 1999 [Page 17]
INTERNET DRAFT August 1998
between DIA1 and DIA2:
<DIAMETER Message> ::= <DIAMETER Header>
<Command AVP>
[<Additional AVPs>]
<Timestamp AVP>
<Nonce AVP>
<Digital-Signature AVP>
<Integrity-Check-Vector AVP (DIA1->DIA2)>
The following message would be sent between DIA2 and DIA3:
<DIAMETER Message> ::= <DIAMETER Header>
<Command AVP>
[<Additional AVPs>]
<Timestamp AVP>
<Nonce AVP>
<Digital-Signature AVP>
<Timestamp AVP>
<Nonce AVP>
<Integrity-Check-Vector AVP (DIA2->DIA3)>
Note that in the above example messages the ICV AVP appear after the
Digital-Signature AVP. This is necessary since DIA2 above removes the
ICV AVP (DIA1->DIA2) and adds its own ICV AVP (DIA2->DIA3). The ICVs
provide hop-by-hop security while the Digital-Signature provides
integrity of the message between DIA1 and DIA3.
<Shared-Secret> <Public-Key>
+------+ -----> +------+ -----> +------+
| | | | | |
|router+----------+ DIA1 +----------+ DIA2 |
| | | | | |
+------+ <----- +------+ <----- +------+
There are cases, such as in remote access, where the device
initiating the DIAMETER request does not have the processing power to
generate Digital-Signatures as required by the protocol. In such an
arrangement, there normally exists a first hop DIAMETER Server (DIA1)
which acts as a proxy to relay the request to the final
authenticating DIAMETER server (DIA2). It is valid for the first hop
server to remove the Integrity-Check-Vector AVP inserted by the
router and replace it with a Digital-Signature AVP.
4.4 AVP Encryption with Public Keys
AVP encryption using public keys is much more complex than the
Calhoun, Bulley expires February 1999 [Page 18]
INTERNET DRAFT August 1998
previously decribed method, yet it is desirable to use it in cases
where the DIAMETER message will be processed by an untrusted
intermediate node (proxy).
Public Key encryption SHOULD be supported, however it is permissible
for a low powered device initiating the DIAMETER message to use
shared secret encryption with the first hop (proxy) DIAMETER server,
which would decrypt and encrypt using the Public Key method.
The PK-Encrypted-Data bit MUST only be set if the final DIAMETER host
is aware of the sender's public key. This information can be relayed
in three different methods as described in section 4.3.
The AVP is encrypted in the method described in [9].
4.5 Public Key Cryptography Support
A DIAMETER peer's public key is required in order to validate a
message which includes the the Digital-Signature AVP. There are three
possibilities on retrieving public keys:
4.5.1 X509-Certificate
A message which includes a Digital-Signature MAY include the X509-
Certificate AVP. Given the size of a typical certificate, this is
very wasteful and in most cases DIAMETER peers would cache such
information in order to minimize per packet processing overhead.
It is however valid for a DIAMETER host to provide its X509-
Certificate in certain cases, such as when issuing the Device-
Reboot-Indication. It is envisioned that the peer would validate and
cache the certificate at that time.
4.5.2 X509-Certificate-URL
The X509-Certificate-URL is a method for a DIAMETER host sending a
message that includes the Digital-Signature to provide a pointer to
its certificate.
Upon receiving such a message a DIAMETER host MAY choose to retrieve
the certificate if it is not locally cached. Of course the process of
retrieving and validating a certificate is lengthy and will require
the initiator of the message to retransmit the request. However once
cached the certificate can be used until it expires.
Calhoun, Bulley expires February 1999 [Page 19]
INTERNET DRAFT August 1998
4.5.3 Static Public Key Configuration
Given that using certificates requires a PKI infrastructure which is
very costly, it is also possible to use this technology by locally
configuring DIAMETER peers' public keys.
Note that in a network involving many DIAMETER proxies this may not
scale well.
5.0 References
[1] Rigney, et alia, "RADIUS", RFC-2138, April 1997
[2] Calhoun, Rubens, "DIAMETER Base Protocol", Internet-Draft,
draft-calhoun-diameter-05.txt, August 1998.
[3] Rivest, Dusse, "The MD5 Message-Digest Algorithm",
RFC 1321, April 1992.
[4] Kaufman, Perlman, Speciner, "Network Security: Private
Communications in a Public World", Prentice Hall,
March 1995, ISBN 0-13-061466-1.
[5] Krawczyk, Bellare, Canetti, "HMAC: Keyed-Hashing for Message
Authentication", RFC 2104, January 1997.
[6] Calhoun, Bulley, "DIAMETER User Authentication Extensions",
draft-calhoun-diameter-authen-03.txt, May 1998.
[7] Aboba, Beadles "The Network Access Identifier." RFC 2486.
January 1999.
[8] Aboba, Zorn, "Criteria for Evaluating Roaming Protocols",
RFC 2477, January 1999.
[9] Kaliski, "PKCS #1: RSA Encryption Version 1.5", Internet-
Draft, draft-hoffman-pkcs-rsa-encrypt-03.txt, October 1997.
[10] Calhoun, Zorn, Pan, "DIAMETER Framework",
draft-calhoun-diameter-framework-02.txt, Work in Progress,
December 1998
[11] Aboba, Vollbrecht, "Proxy Chaining and Policy Implementation in
Roaming", draft-ietf-roamops-auth-10.txt, Work in Progress,
February 1999.
6.0 Acknowledgements
The Authors would like to acknowledge the following people for their
contribution in the development of the DIAMETER protocol:
Bernard Aboba, Jari Arkko, , Daniel C. Fox, Lol Grant, Nancy Greene,
Peter Heitman, Ryan Moats, Victor Muslin, Kenneth Peirce, Allan
Rubens, Sumit Vakil, John R. Vollbrecht, Jeff Weisberg and Glen Zorn
7.0 Author's Address
Calhoun, Bulley expires February 1999 [Page 20]
INTERNET DRAFT August 1998
Questions about this memo can be directed to:
Pat R. Calhoun
Technology Development
Sun Microsystems, Inc.
15 Network Circle
Menlo Park, California, 94025
USA
Phone: 1-650-786-7733
Fax: 1-650-786-6445
E-mail: pcalhoun@eng.sun.com
William Bulley
Merit Network, Inc.
4251 Plymouth Road, Suite C
Ann Arbor, Michigan, 48105-2785
USA
Phone: 1-734-764-9993
Fax: 1-734-647-3185
E-mail: web@merit.edu
Calhoun, Bulley expires February 1999 [Page 21]